CHAIN LENGTH-DEPENDENT PB(II)-COORDINATION IN PHYTOCHELATINS

UV/visible and circular dichroism (CD) spectroscopy have been used to study the binding of Pb(II) to plant metal-sequestering peptides, phytochelatins (PCs), with the structure (gamma Glu-Cys)(2)Gly, (gamma Glu-Cys)(3)Gly and (gamma Glu-Cys)(4)Gly. Saturation of the Pb(II)-induced charge-transfer bands indicated that both(gamma Glu-Cys)(2)Gly and (gamma Glu-Cys)(3)Gly bound one metal ion per peptide molecule. However, (gamma Glu-Cys)(4)Gly formed two distinct species with stoichiometries of one and two Pb(II) ions per peptide molecule, respectively. The optical spectra of Pb(II)(1)-(gamma Glu-Cys)(4)Gly were similar to those of Pb(II)(1)-(gamma Glu-Cys)(3)Gly, whereas the spectra of Pb(II)(2)-(gamma Glu-Cys)(4)Gly were similar to those of Pb(II)(1)-(gamma Glu-Cys)(2)Gly. Since cysteinyl thiolates are the likely ligands for Pb(II) in PCs, Pb(II) appears to form two-, three- and four-coordinate complexes with PCs depending on their chain length. Furthermore, Pb(II) may exhibit multiple coordination in longer chain PCs as indicated by the formation of two Pb(II)-binding species of (gamma Glu-Cys)(4)Gly. The transfer of Pb(II) from glutathione to PCs and from shorter chain to longer chain PCs is also demonstrated. (C) 1995 Academic Press, Inc.